A liquid crystal display device is provided with a backlight unit that applies light to the back side of a liquid crystal panel (LCD) so as to illuminate it. The backlight unit is composed of such components as an illuminating light source, a light guide plate that diffuses light emitted from the light source so that it is applied to the liquid crystal panel, a prism sheet or diffusion sheet for rendering the light emitted from the light guide plate to become uniform.
A known example of such backlight unit is disclosed in Patent Document 1.
FIG. 25 is a schematic sectional view of an area light source device disclosed in Patent Document 1.
The area light source device (backlight unit) shown in FIG. 25 is formed by embedding fluorescent lamps 102 in a light guide plate 100, then providing a reflective sheet 104 on a rear surface of the light guide plate 100 and superposing a transmitted light amount correcting sheet 106, a light diffuser 108, and a prism sheet 110 on the light emission plane of the light guide plate 100.
The light guide plate 100 has a substantially rectangular shape and is made of a resin having dispersed therein fine particles that can diffuse illuminating light. In addition, the light guide plate 100 has a flat top surface, which is used as the emission plane. Further, grooves 100a each having a U-shaped cross section are formed in the rear surface (away from the emission plane) of the light guide plate 100 such that the fluorescent lamps 102 can be fitted into those grooves. A light amount correcting surface 100b for promoting the emission of illuminating light is formed on the emission plane of the light guide plate 100, except in portions just above the fluorescent lamps 102.
As mentioned above, Patent Document 1 shows that not only by forming the light guide plate 100 with the fine particles mixed therein but also by promoting the emission of the illuminating light with the aid of the light amount correcting surface 100b formed on a part or all of the emission plane except in the portions just above the fluorescent lamps 102, the total thickness of the device can be made small enough while at the same time any unnatural luminance irregularities that might occur to the emitting light can be reduced.
In order to produce a backlight for liquid crystal display devices that enables the liquid crystal display device to be reduced in size, weight and thickness while permitting smaller cost and lower power consumption and yet without reducing the amount of light applied from the backlight, Patent Document 2 discloses a light guide plate having a rectangular surface for light application, a groove having a rectangular cross section which is gouged out in the central portions of the shorter sides to extend in parallel with the longer sides and in which a light source is to be inserted, and a rear surface formed such that the thickness of the plate gradually decreases from the groove toward both lateral surfaces on the longer sides.
In order to produce a backlight unit that enables a liquid crystal display device to have a narrow enough frame and a small enough thickness to assure brightness with high utilization of light, Patent Document 3 discloses a light guide member (light guide plate) having such a concave portion for accommodating a light source that its cross section as taken parallel to the width direction is of a parabolic shape with the major axis running in the depth direction.
In addition, Patent Document 4 discloses a light guide plate which, in order to ensure that the in-plane brightness of a display panel is kept uniform to provide illumination of high luminance, places a plurality of plate-like light waveguide layers in superposition on a substantially V-shaped high-reflection layer so as to provide an increasing scale of refractive index and uses the light emitted from the light emitting end face of each waveguide layer in order to realize a brighter light diffusion layer. The concave portion formed in the light guide plate to accommodate the light source is of a triangular shape.
However, the light guide plates described above all suffer the problem of causing luminance irregularities to the illuminating light emitting from the emission plane.
For example, the light guide plate 100 disclosed in Patent Document 1 has the light sources (fluorescent lamps) 102 fitted into the grooves 100a with a circular cross section and, as shown in FIG. 26, the luminance peaks due to the light sources 102 remain unresolved.
In view of this, in order that it can be used as a planar light source device, the light guide plate 100 has the transmitted light amount correcting sheet 106, the light diffuser 108, and the prism sheet 110 provided on a side closer to its emission plane, thereby removing unnatural luminance irregularities that will occur on the emission plane.
The transmitted light amount correcting sheet 106 is prepared by forming a tiny dotted pattern of metal film over a transparent polyester film. The dotted pattern is formed in such a way that the dot density is highest just above the fluorescent lamps 102 and gradually decreases with the increasing distance from the fluorescent lamps. Designed this way, the transmitted light amount correcting sheet 106 reflects part of the illuminating light emitted by the light guide plate 100 and brings it back into the light guide plate so that the distribution of the quantity of the transmitted light becomes uniform.
The light diffuser 108 is formed of a semitransparent milk-white acrylic plate and it diffuses the light emitted from the transmitted light amount correcting sheet 106 so that it emits with a desired distribution of quantity.
The prism sheet 110 is a light controlling member that corrects the directivity of the emitting light; it is formed of a sheeting of a light-transmitting material such as polycarbonate and has a prism surface formed on the side remote from the light guide plate. The prism surface is comprised of repeated projections with a triangular cross section that extend almost parallel to one another in one direction. It is by the inclined surfaces of these triangular projections that the prism sheet 110 corrects the emitting light to exit such that its primary direction of emission is toward the front side of the emission plane.
Thus, Patent Document 1 claims that by means of the light amount correcting sheet 106, the light diffuser 108, and the prism sheet 110, the irregular light emitting from the emission plane of the light guide plate 100 is diffused to ensure emission of uniform light.
In addition, various methods have been proposed with a view to diffusing light as it emits from the light guide plate (see Patent Documents 5 and 6).
For example, Patent Document 5, as shown in FIG. 10, thereof, discloses a liquid crystal display device having a dotted area printed on a surface of a diffuser plate in order to block light transmission. The printed dots are dense in the region 39A where a cold cathode fluorescent lamp 36 is positioned right beneath it while their density decreases with the increasing distance from that region. This document states that by this design, the quantity of light emitted toward the diffuser plate becomes uniform as it reaches every part of the plate.
Patent Document 6 discloses an area light source device that has a light quantity adjusting layer provided on a surface opposite a linear light source for ensuring that part of the light radiated from the upper surface is reflected and scattered back to a light guide plate. This document states that the light quantity adjusting layer may be formed in such a way that its area percentage decreases with the increasing distance from the linear light source or that it may be formed only in the neighborhood of the linear light source. The document further comments that while the transparent flat plate is typically made of a light-transmitting resin or film, it may be replaced by a light diffusing plate to ensure that the light emitted from the light guide plate and the linear light source toward the upper surface has a smooth intensity distribution.
Patent Document 1: JP 9-304623 A
Patent Document 2: JP 8′-62426 A
Patent Document 3: JP 10-133027 A
Patent Document 4: JP 5-249320 A
Patent Document 5: JP 5-127156 A
Patent Document 6: JP 6-235825 A